The behavior of protein malnourished rhesus monkeys reared under different social conditions was studied extensively for four years with the following general conclusions: (1) No deficiencies in traditional WGTA learning tasks could be detected, except on tests that placed increasing demands upon the animal's attention. These differences disappeared following nutritional rehabilitation. (2)Protein malnourished animals showed a greater interest in food than controls and consistently selected diets containing higher concentrations of protein in two-choice or multiple choice concurrent preference test. (3) Novel stimuli were demonstrated to have motivational consequences in low protein monkeys by inhibiting or interfering with ongoing overt response sequences, and by acting as an energizer in a disinhibition of delay situation. (4) The social behavior of protein malnourished monkeys was typified by a decrease in social contacts, compared to controls. Social encounters by the low protein animals most often resulted in nonreciprocal behaviors and aggression. The incidence and intensity of agonistic behavior appeared to increase in the rehabilitated monkeys. (5) Early social deprivation had a greater effect on the frequency of abnormal social behaviors than nutritional manipulations. Protein deficiency, however, aggravated or intensified some of the deviant behavior patterns observed in impoverished or enriched social animals. (6) Dominance relationships tended to be more unstable in low protein groups than in controls, across a variety of tests. Malnourished monkeys who were submissive and subordinate to high protein animals in shock avoidance competition and social room interactions were found to be more aggressive and competitive than controls in food acquisition.